Transparent bullet-resistant laminates having glass and plastic laminae are well known. Early such laminates, U.S. Pat. No. 2,991,209, contained laminae of a "hard resin material", such as, polymethyl methacrylate, in addition to laminae of glass and were able to stop a greater number of bullets than the conventional glass bullet-resistant panels of the time having twice the weight.
Recent efforts of the bullet resistant laminate industry have likewise been directed towards more stopping power at reduced weight with a growing emphasis placed on thickness and cost. These efforts have resulted in laminates having an increased number of plies, reduced glass content, and increased shock resistant resin content. It has become normal practice in the industry in constructing such laminates to utilize glass or relatively hard solid resinous materials as the impact shock receiving layers while utilizing polycarbonate as a back or inner or downstream layer or that presented to the person or object being protected.
The polycarbonates have become a common shock resistant resin in bullet resistant laminates. U.S. Pat. No. 3,624,238 discloses the use of a polycarbonate lamina as the middle layer of a symmetrical and predominantly glass bullet resistant structure. This particular laminate, by today's standard, has insufficient penetration resistance, is subject to spalling, is too heavy, and is unable to withstand multiple penetrations.
U.S. Pat. No. 3,666,614 discloses laminated assemblies for use as structural windshields in motor vehicles. These laminated assemblies are glass-polycarbonate laminates, for example, two ply laminates having a forward facing lamina of glass and a back lamina of polycarbonate bonded with an adhesive interlayer of ethylene-vinyl acetate. Optionally, primers may be used to increase adhesion of the laminae and a mar-resistant coating may be applied to the external face of the polycarbonate lamina. Embodiments of this invention suffer from, among other deficiencies, a tendency of the back polycarbonate lamina to spall upon impact of an object with the front glass lamina and an insufficient level of penetration resistance to projectiles.
U.S. Pat. No. 4,328,277, assigned to the same assignee as the present invention, discloses a bullet resistant laminate having laminae of a solid resinous material such as polycarbonate or glass, adhesive interlayers of a compatible adhesive such as polydiorganosiloxane-polycarbonate block copolymer, and a back mar-resistant polycarbonate lamina of controlled thickness opposite the direction of impact or shock. The disclosed embodiments of this reference provide excellent penetration resistance for their weight and thickness. However, in some situations, it may be desirable to provide greater penetration resistance, in order to stop very high speed projectiles especially if such can be done without substantial increases in weight or thickness.
U.S. Pat. No. 4,123,588, assigned to the same assignee as the present invention, discloses a bullet resistant laminate having at least one lamina of acrylic polymer, a lamina or laminae of glass or a solid resinous material such as polycarbonate, and at least one polydiorganosiloxane-polycarbonate block copolymer adhesive interlayer. As above, the preferred embodiments of this invention provide excellent penetration resistance for their size and thickness, but in some situations, greater penetration resistance is necessary in order to stop very high speed projectiles.
Thus, reasonably lightweight and thin laminates exist having excellent multi-projectile stopping power without spall, but these laminates are most effective against bullet sized projectiles having medium range velocities on the order of less than 600 m/sec. These laminates are substantially less effective against such projectiles at very high speeds, that is speeds on the order of 600 m/sec. and greater.
Therefore, it is an object of the present invention to produce an impact or shock resistant laminate having improved penetration resistance without substantial weight gain or increased thickness.
It is another object of the present invention to produce an impact or shock resistant laminate capable of withstanding multiple penetrations by bullet size projectiles having velocities in excess of 600 m/sec.